A fluorinated alloy-type interfacial layer enabled by metal fluoride nanoparticle modification for stabilizing Li metal anodes

High Rate and Low-Temperature Stable Lithium Metal Batteries Enabled by Lithiophilic 3D Cu-CuSn Porous Framework

Lithium (Li) metal is regarded as the "Holy Grail" of anodes for high-energy rechargeable lithium batteries by virtue of its ultrahigh theoretical specific capacity and the lowest redox potential. However, the Li dendrite impedes the practical application of Li metal anodes. Herein, lithiophilic three-dimensional Cu-CuSn porous framework (3D Cu-CuSn) was fabricated by a vapor phase dealloying strategy via the difference in saturated vapor pressure between different metals and the Kirkendall effect. CuSn alloy sites were converted into LiSn alloy sites through the molten Li infusion method, and composite Li metal anodes (3D Cu-LiSn-Li) are achieved. Alloyed tin, as the bridge between the porous copper substrate and metallic Li, plays a critical role in optimizing Li nucleation and enhancing the fast lithium migration kinetics. This work demonstrates that lithiophilic binary copper alloys are an effective way to achieve room-temperature high rate performance and satisfied low-temperature cycling stability for Li metal batteries.

Nucleophilic deposition behavior of metal anodes

Nucleophilic materials play important roles in the deposition behavior of high-energy-density metal batteries (Li, Na, K, Zn, and Ca), while the principle and determination method of nucleophilicity are lacking. In this review, we summarize the metal extraction/deposition process to find out the mechanism of nucleophilic deposition behavior. The key points of the most critical nucleophilic behavior were found by combining the potential change, thermodynamic analysis, and active metal deposition behavior. On this basis, the inductivity and affinity of the material have been determined by Gibbs free energy directly. Thus, the inducibility of most materials has been classified: (a) induced nuclei can reduce the overpotential of active metals; (b) not all materials can induce active metal deposition; (c) the induced reaction is not changeless. Based on these results, the influencing factors (temperature, mass, phase state, induced reaction product, and alloying reactions) were also taken into account during the choice of inducers for active metal deposition. Finally, the critical issues, challenges, and perspectives for further development of high-utilization metal electrodes were considered comprehensively.

Design Principles for Fluorinated Interphase Evolution via Conversion-Type Alloying Processes for Anticorrosive Lithium Metal Anodes

Over the past decade, lithium metal has been considered the most attractive anode material for high-energy-density batteries. However, its practical application has been hindered by its high reactivity with organic electrolytes and uncontrolled dendritic growth, resulting in poor Coulombic efficiency and cycle life. In this paper, we propose a design strategy for interface engineering using a conversion-type reaction of metal fluorides to evolve a LiF passivation layer and Li-M alloy. Particularly, we propose a LiF-modified Li-Mg-C electrode, which demonstrates stable long-term cycling for over 2000 h in common organic electrolytes with fluoroethylene carbonate (FEC) additives and over 700 h even without additives, suppressing unwanted side reactions and Li dendritic growth. With the help of phase diagrams, we found that solid-solution-based alloying not only facilitates the spontaneous evolution of a LiF layer and bulk alloy but also enables reversible Li plating/stripping inward to the bulk, compared with intermetallic compounds with finite Li solubility.

In situ artificial solid electrolyte interface engineering on an anode for prolonging the cycle life of lithium-metal batteries

Lithium, with its high theoretical capacity and low potential, has been widely investigated as the anode in energy storage/conversion devices. However, their commercial applications always suffer from undesired dendrite growth, which forms in the charging process and may puncture the separator, leading to short cycle lives and even security problems. Herein, by an in situ displacement reaction using SnF₂ at room temperature, we constructed an artificial solid electrolyte interface (ASEI) of LiF/Li-Sn outside the Li anode. This hybrid strategy can induce a synergy between the high Li⁺ conductivity of the Li-Sn alloy and good electrical insulation of LiF. Moreover, extreme synergy can be achieved by moderating the thickness of the LiF/Li-Sn ASEI, guiding dendrite-free lithium plating and stripping. As a result, a Li//LiFePO₄ battery that is assembled from the LiF/Li-Sn ASEI-engineered Li anode can obtain 1000 cycled lives with 86.3% capacity retention under a charge/discharge rate of 5 C. This work provides an alternative way to construct dendrite-free lithium metal anodes, which significantly benefit the cycle lives of LMBs.

Germanium-Free Dense Lithium Superionic Conductor and Interface Re-Engineering for All-Solid-State Lithium Batteries against High-Voltage Cathode

Li₁₀GeP₂S₁₂ (LGPS) solid electrolyte is not affordable due to the high cost of Ge metal, making it economically unviable despite being a lithium superionic conductor. The synthesis of such solid electrolytes is much more time- and energy-consuming and needs an inert environment. Here, we report Si (silicon)-based composition [Li₁₀SiP₂S₁₂ (LSiPS)] to make it cost-effective through microwave heating (MW). The total time for synthesis processes, including ball milling, heating rate, and heating dwell time, is ∼120 min, much less than the previous reports. We have also avoided vacuum sealing/Ar-purging to reduce the synthesis cost further. During MW heating, the densification process dominates over coarsening, resulting in a dense nanoflake morphology with a finer crystallite size. The synthesized LSiPS has a high fraction (∼89%) of more conducting tetragonal phase as identified by NMR analysis. Further, we modified the interface between the Li anode and LSiPS by forming a lithiophobic and lithiophilic kind of gradient interlayer to reduce the reduction of LSiPS and suppress the side reactions. The interface modification resulted in a better Li/LSiPS/Li cyclic performance for 1800 h at 0.2 mA/cm² and 500 h at 1.0 mA/cm². All-solid-state lithium-metal batteries (ASSLIB) have been developed against a high-voltage cathode (LCMO-coated LCO) and showed an excellent cycling performance with a reversible capacity of ∼110 mAh/g after 300 cycles.

Slidable and Highly Ionic Conductive Polymer Binder for High-Performance Si Anodes in Lithium-Ion Batteries

Silicon is expected to become the ideal anode material for the next generation of high energy density lithium battery because of its high theoretical capacity (4200 mAh g^-1 ). However, for silicon electrodes, the initial coulombic efficiency (ICE) is low and the volume of the electrode changes by over 300% after lithiation. The capacity of the silicon electrode decreases rapidly during cycling, hindering the practical application. In this work, a slidable and highly ionic conductive flexible polymer binder with a specific single-ion structure (abbreviated as SSIP) is presented in which polyrotaxane acts as a dynamic crosslinker. The ionic conducting network is expected to reduce the overall resistance, improve ICE and stabilize the electrode interface. Furthermore, the introduction of slidable polyrotaxane increases the reversible dynamics of the binder and improves the long-term cycling stability and rate performance. The silicon anode based on SSIP provides a discharge capacity of ≈1650 mAh g^-1 after 400 cycles at 0.5C with a high ICE of upto 92.0%. Additionally, the electrode still exhibits a high ICE of 87.5% with an ultra-high Si loading of 3.84 mg cm^-2 and maintains a satisfying areal capacity of 5.9 mAh cm^-2 after 50 cycles, exhibiting the potential application of SSIP in silicon-based anodes.

Room-Temperature Anode-Less All-Solid-State Batteries via the Conversion Reaction of Metal Fluorides

All-solid-state batteries (ASSBs) that employ anode-less electrodes have drawn attention from across the battery community because they offer competitive energy densities and a markedly improved cycle life. Nevertheless, the composite matrices of anode-less electrodes impose a substantial barrier for lithium-ion diffusion and inhibit operation at room temperature. To overcome this drawback, here, the conversion reaction of metal fluorides is exploited because metallic nanodomains formed during this reaction induce an alloying reaction with lithium ions for uniform and sustainable lithium (de)plating. Lithium fluoride (LiF), another product of the conversion reaction, prevents the agglomeration of the metallic nanodomains and also protects the electrode from fatal lithium dendrite growth. A systematic analysis identifies silver (I) fluoride (AgF) as the most suitable metal fluoride because the silver nanodomains can accommodate the solid-solution mechanism with a low nucleation overpotential. AgF-based full cells attain reliable cycling at 25 °C even with an exceptionally high areal capacity of 9.7 mAh cm^-2 (areal loading of LiNi_0.8 Co_0.1 Mn_0.1 O₂  = 50 mg cm^-2 ). These results offer useful insights into designing materials for anode-less electrodes for sulfide-based ASSBs.

Transferring Liquid Metal to form a Hybrid Solid Electrolyte via a Wettability-Tuning Technology for Lithium-Metal Anodes

Integrating solid-state electrolyte (SSE) into Li-metal anodes has demonstrated great promise to unleash the high energy density of rechargeable Li-metal batteries. However, fabricating a highly cyclable SSE/Li-metal anode remains a major challenge because the densification of the SSE is usually incompatible with the reactive Li metal. Here, a liquid-metal-derived hybrid solid electrolyte (HSE) is proposed, and a facile transfer technology to construct an artificial HSE on the Li metal is reported. By tuning the wettability of the transfer substrates, electron- and ion-conductive liquid metal is sandwiched between electron-insulating and ion-conductive LiF and oxides to form the HSE. The transfer technology renders the HSE continuous, dense, and uniform. The HSE, having high ion transport, electron shut-off, and mechanical strength, makes the composite anode deliver excellent cyclability for over 4000 h at 0.5 mA cm^-2 and 1 mAh cm^-2 in a symmetrical cell. When pairing with LiFePO₄ and sulfur cathodes, the HSE-coated Li metal dramatically enhances the performance of full cells. Therefore, this work demonstrates that tuning the interfacial wetting properties provides an alternate approach to build a robust solid electrolyte, which enables highly efficient Li-metal anodes.

Importance of Halide Ions in the Stabilization of Hybrid Sn-Based Coatings for Lithium Electrodes

The properties of hybrid Sn-based artificial solid electrolyte interphase (SEI) layers in protecting Li-metal electrodes toward surface instabilities were investigated via a combined experimental and theoretical approach. The performance of coating layers can be coherently explained based on the nature of the coating species. Notably, when starting from a chloride precursor, the hybrid coating layer is formed by an intimate mixture of Li₇Sn₂ and LiCl: the first ensures a high bulk ionic conductivity, while the second forms an external layer allowing a fast surface diffusion of Li⁺ to avoid dendrite growth, a low surface tension to guarantee the thermodynamic stability of the protective layer, and a negative underneath plating energy (UPE) to promote lithium plating at the interface between the Li metal and the coating layer. The synergy between the two components and, in particular, the crucial role of LiCl in the promotion of such an underneath plating mechanism are shown to be the key properties to improve the performance of artificial SEI layers.

Lithium reduction reaction for interfacial regulation of lithium metal anode

The lithium metal anode (LMA) is regarded as a very promising candidate for next-generation lithium batteries. The interfacial issue plays a pivotal role in affecting the lithium plating/stripping behavior, Coulombic efficiency and cycling lifespan of an LMA. The lithium reduction reaction (LRR) is an advanced regulating technique for optimizing the LMA interphase, which intelligently utilizes lithium metal itself as an interphase precursor. This strategy also possesses moderate operating conditions, high efficiency, great convenience and scalability. In this review, the latest developments of LRRs in interfacial regulation for LMAs are summarized, focusing on the interfacial regulation mechanism and the construction of various inorganic/organic interfaces in lithium metal liquid/solid batteries. The target interface properties and corresponding influence factors during LRRs are investigated in detail. Besides this, the superiority and insufficiency of LRRs are discussed and possible directions for LRRs are presented. This review highlights in situ modification characteristics for anode interface regulation during the LRR and can be extended to other metal anodes such as sodium, potassium and zinc.

Engineering and characterization of interphases for lithium metal anodes

Lithium metal is a very promising anode material for achieving high energy density for next generation battery systems due to its low redox potential and high theoretical specific capacity of 3860 mA h g-1. However, dendrite formation and low coulombic efficiency during cycling greatly hindered its practical applications. The formation of a stable solid electrolyte interphase (SEI) on the lithium metal anode (LMA) holds the key to resolving these problems. A lot of techniques such as electrolyte modification, electrolyte additive introduction, and artificial SEI layer coating have been developed to form a stable SEI with capability to facilitate fast Li+ transportation and to suppress Li dendrite formation and undesired side reactions. It is well accepted that the chemical and physical properties of the SEI on the LMA are closely related to the kinetics of Li+ transport across the electrolyte-electrode interface and Li deposition behavior, which in turn affect the overall performance of the cell. Unfortunately, the chemical and structural complexity of the SEI makes it the least understood component of the battery cell. Recently various advanced in situ and ex situ characterization techniques have been developed to study the SEI and the results are quite interesting. Therefore, an overview about these new findings and development of SEI engineering and characterization is quite valuable to the battery research community. In this perspective, different strategies of SEI engineering are summarized, including electrolyte modification, electrolyte additive application, and artificial SEI construction. In addition, various advanced characterization techniques for investigating the SEI formation mechanism are discussed, including in situ visualization of the lithium deposition behavior, the quantification of inactive lithium, and using X-rays, neutrons and electrons as probing beams for both imaging and spectroscopy techniques with typical examples.